The present invention relates to a novel agricultural chemical composition containing a hydrolyzate of chitosan and acetic acid useful for protecting various plants from harmful organisms or preventing plant diseases caused by harmful organisms as well as to a method for preventing plant diseases caused by harmful organisms by using the chemical composition.
As is known, chitosan is a deacetylation product of chitin contained in the crusts of various crustacean animals such as crabs and lobsters and cell walls of certain microorganisms. Chitosan can be hydrolyzed to give a low molecular weight chitosan which is referred to as a chitosan hydrolyzate hereinbelow.
It is known that chitosan and chitosan hydrolyzates have influences on the growth of various phytopathogenic fungi as reported in Experimental Mycology, volume 3, pages 285-287 (1979), Phytopathologische Zeitschrift, volume 111, pages 82-89 (1984), Experimental Mycology, volume 8, pages 276-281 (1984) and Agricultural and Biological Chemistry, volume 53, pages 3065-3066 (1989) and a proposal is made for the utilization thereof as an agricultural chemical agent for preventing black spot of pear in practice (see Japanese patent Kokai No. 62-198604).
The agricultural chemical agents containing chitosan or a chitosan hydrolyzate heretofore proposed have several problems that the effectiveness thereof is limited to several particular plant diseases and that damages to the plants caused by the chemicals is unavoidable when the agent is used in high concentrations. Accordingly, it is eagerly desired to develop an agricultural chemical agent having a broad anti-microbial spectrum to exhibit effectiveness against various plant diseases without limitation and capable of exhibiting the desired effect by using in a low concentration free from the problem of damage to plants due to the chemicals.
On the other hand, it is also known that acetic acid and other organic acids have a microbicidal action and these organic acids can be used as an active ingredient in microbicides utilizing the microbicidal action thereof. In the field of food industry in which the problem of chemical injury is of relatively low importance as compared with agriculture treating living plants, in particular, organic acids are under practical use as a microbicide or microbiostat.
In the microbicides and microbiostats containing an organic acid, however, it is essential to maintain a high concentration of the organic acid as the active ingredient while, when an agricultural chemical agent containing an organic acid in a concentration high enough to exhibit the microbicidal action is applied to a living plant, serious chemical injuries are caused in the plant so that applicability of such an agent to agriculture is questionable. Furthermore, it is impossible to directly apply acetic acid in a high concentration to exhibit the microbicidal action to plants having relatively low resistance against chemical injuries because acetic acid is one of the organic acids which exhibits particularly serious chemical injuries to plants.
As to the application of the supernatant liquor obtained by the deproteinization of green alfalfa leaves, which is referred to as a brown juice hereinbelow, to agricultural plants, on the other hand, an attempt has been made to utilize the same as a fertilizer although the attempt has not led to establishment of the technology. Moreover, absolutely nothing has been thought of the utilization of the brown juice as an agricultural chemical agent for protecting plants from harmful organisms.
Further, nothing has been taught concerning pest control by using chitosan or chitosan hydrolyzates and organic acids such as acetic acid.